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United States Patent |
5,225,244
|
Aharoni
,   et al.
|
July 6, 1993
|
Polymeric anti-reflection coatings and coated articles
Abstract
Solid bodies having a reflective surface are provided with an
anti-reflection coating of a terpolymer composition derived from (a)
perfluoroalkylalkyl acrylate or methacrylate, (b) acrylic, methacrylic or
itaconic acid, and (c) hydroxyl- containing acrylate or methacrylate.
Inventors:
|
Aharoni; Shaul M. (Morris Plains, NJ);
McFarland; Michael J. (Washington, NJ);
Nahata; Ajay (Chatham, NJ);
Yardley; James T. (Morristown, NJ)
|
Assignee:
|
Allied-Signal Inc. (Morris Township, NJ)
|
Appl. No.:
|
930886 |
Filed:
|
August 14, 1992 |
Current U.S. Class: |
427/240; 427/385.5; 427/407.1; 427/430.1 |
Intern'l Class: |
B05D 003/12 |
Field of Search: |
427/240,385.5,407.1,430.1
|
References Cited
U.S. Patent Documents
3491169 | Jan., 1970 | Raynolds et al.
| |
3547856 | Dec., 1970 | Tandy et al.
| |
3910187 | Oct., 1975 | Cords.
| |
4130706 | Dec., 1978 | Plambeck, Jr.
| |
4293674 | Oct., 1981 | Andrews.
| |
4296224 | Oct., 1981 | Fukui et al.
| |
4644043 | Feb., 1987 | Ohmori et al.
| |
4650843 | Mar., 1987 | Yokoyama et al.
| |
4791166 | Dec., 1988 | Saukaitis.
| |
4812337 | Mar., 1989 | Sugimura et al.
| |
4833207 | May., 1989 | Kinaga et al.
| |
Foreign Patent Documents |
0121140 | Oct., 1984 | EP.
| |
0182516 | May., 1986 | EP.
| |
0234601 | Sep., 1987 | EP.
| |
2151035 | Feb., 1973 | DE.
| |
02155133 | May., 1973 | FR.
| |
44-851 | Jan., 1969 | JP.
| |
56-118408 | Sep., 1981 | JP.
| |
57-51705 | Mar., 1982 | JP.
| |
60-258218 | Dec., 1985 | JP.
| |
Primary Examiner: Schofer; Joseph L.
Assistant Examiner: Sarofim; N.
Attorney, Agent or Firm: Fuchs; Gerhard H., Stewart; Richard C.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This application is a division of application Ser. No. 783,032, filed Oct.
25, 1991, now U.S. Pat. No. 5,178,955.
Claims
We claim:
1. The method of providing an anti-reflection coating on a solid body
having a reflective surface, which comprises depositing on said surface a
layer of a polymeric composition comprising fluorinated copolymer having a
polymer chain composed of
##STR6##
units, wherein s, t and u represent weight proportions of the respective
X, Y and Z units, and have values within the ranges of
s=from about 0.5 to about 0.99;
t=from about 0.005 to about 0.495; and
u=from about 0.005 to about 0.495;
with the sum of s+t+u being 1;
X represents units of the composition
##STR7##
wherein R.sup.1 is H, --CH.sub.3, or mixtures thereof;
p is 1 or 2;
n is an integer of from about 1 to about 40;
Y represents units of the composition
##STR8##
wherein R.sup.2 is H, --CH.sub.3 or --CH.sub.2 COOH;
Z represents units of the composition
##STR9##
wherein R.sup.3 is H, --CH.sub.3, or CH.sub.2 COOC.sub.m H.sub.2m+1,
wherein m is an integer of from about 1 to about 4, and `R.sup.4 is an
alkylene bridging group, straight chain, branched or cyclic, having from 1
to about 8 carbon atoms;
wherein the X, Y and Z units may be arranged in any sequence.
2. The method of claim 1 further comprising heating said layer of a
polymeric composition at a temperature between about 120.degree. C. and
about 180.degree. C. for time sufficient to effect cross-linking of said
fluorinated copolymer.
3. The method of making an anti-reflection coated body according to claim
1, which comprises applying a solution of said fluorinated copolymer to
said reflective surface, and evaporating the solvent.
4. The method of claim 3, further comprising heating the resultant coating
at a temperature between about 120.degree. C. and about 180.degree. C. for
time sufficient to effect cross-linking of said fluorinated copolymer.
5. The method of claim 3, wherein the solution of said copolymer is applied
by spin-coating.
6. The method of claim 3, wherein the solution of said copolymer is applied
by means of dipping said solid body into said solution.
7. The method of claim 4, wherein said solid body is transparent.
8. The method of claim 1, comprising the steps of depositing two layers of
anti-reflection coatings, wherein the first layer which is in direct
contact with said body comprises a polymer having a refractive index
higher than the refractive index of said body, and the second layer has a
lower refractive index than said body.
9. The method of claim 8 involving the step of applying a solution of said
fluorinated copolymer to the first layer, and evaporating the solvent.
10. The method of claim 9 further comprising heating the second layer at
temperature between about 120.degree. C. and about 180.degree. C. for time
sufficient to effect cross-linking of the fluorinated copolymer.
Description
FIELD OF THE INVENTION
This invention relates to the field of optical coatings for reducing
reflection on reflective surfaces, such as optical surfaces, windows,
transparent films, display surfaces, glossy photographs and the like. It
provides coatings of curable optically clear fluoropolymer having low
refractive index.
BACKGROUND OF THE INVENTION
In any optical element, a percentage of the incident light is reflected at
each surface. The exact amount is determined by the refractive index
change at the dielectric interface. There are four main reasons why
anti-reflection coatings are desirable in optical systems. First, the
percentage of reflected light is lowest at normal incidence. The
percentage increases with a corresponding increase in the angle of
observation. This limits the resolution of the image and, in some cases,
can completely obscure the image. Second, an increase in reflected light
corresponds to a decrease in transmitted light. For components such as
compound lenses, this loss is multiplicative and may be intolerable.
Third, reflections from optical surfaces often create unwanted or
distracting glare. Finally, for components such as camera lenses and
photographs, that contain many optical surfaces, there are multiple
internal reflections. These reflections can cause stray light to hit the
image plane and thereby reduce the image contrast and definition.
It has been well known for many years that unwanted reflections can be
substantially reduced by providing a surface coating of an optically clear
coating material having a refractive index which is lower than the
refractive index of the substrate. However, difficulty in producing high
quality thin films prevented significant practical application until
approximately 1940, when the technology for the creation of thin films of
various refractory inorganic materials via evaporation under high vacuum
conditions was developed. In more recent times, low refractive index
polymeric coatings, generally fluoropolymer coatings, have been provided
for anti-reflection applications. Generally, these coatings, for maximum
effectiveness, are about 1/4 wavelength thick. The basic theory of such
anti-reflection coatings is well known; the technical challenge is in the
provision of conveniently applied, effective, strongly adherent,
scratch-resistant and relatively low cost coatings with optimally low
refractive index.
SUMMARY OF THE INVENTION
This invention provides devices comprising a reflective substrate having
deposited thereon as an anti-reflection coating a thin film of copolymer
of fluorine-containing acrylic monomers with non-fluorinated acrylic
monomers. Such copolymer can be made under free-radical polymerization
conditions. These copolymers are amorphous and optically clear, and have
low refractive indexes. Being soluble in specific organic solvents, their
solutions can be used to cast anti-reflection coatings, which can be cured
by cross-linking. These coatings are strongly adherent to substrates,
including glass, polymers, polymer films and crystal substrates. These
copolymer anti-reflection coatings combine the superior properties of
fluoropolymers--such as low refractive index and surface energy, good
thermal and chemical resistance--with strong adhesion, flexibility,
toughness, and abrasion resistance. Moreover, they can be easily applied
from solution, and they are readily cured by cross-linking. They are
suitable for application to very large area substrates at relatively low
cost.
In accordance with the present invention, there are provided devices
comprising a reflective substrate having deposited thereon as an
anti-reflection coating an effective layer of a polymeric composition
comprising fluorinated copolymer having the general composition
##STR1##
wherein R.sup.1 is H, --CH.sub.3, or mixtures thereof;
R.sup.2 is H, --CH.sub.3, or --CH.sub.2 COOH;
R.sup.3 is H, --Ch.sub.3, or --CH.sub.2 COOC.sub.m H.sub.2m+1,
wherein m is an integer of from about 1 to about 4;
R.sup.4 is an alkylene bridging group, straight chain, branched or cyclic,
having from 1 to about 8 carbon atoms;
p is 1 or 2;
s, t and u represent weight proportions of the respective monomer-derived
units, and have values within the ranges of
s=from about 0.5 to about 0.99;
t=from about 0.005 to about 0.495; and
u =from about 0.005 to about 0.495;
with the sum of s+t+u being 1; and
n is an integer of from about 1 to about 40;
wherein the monomer-derived units may be arranged in any sequence. In the
above formula, t and u may, but need not be the same.
The term copolymer, as used in the specification and claims, is intended to
refer to a polymer derived from at least two or more, usually derived from
at least three different monomer units. There is no theoretical limit on
the number of different monomer units which may be incorporated into the
polymeric compositions forming the anti-reflection coatings for the
optical devices of the present invention; their number is limited only by
the usual practical limitations imposed by polymerization process
considerations, and the desire to obtain polymer products having useful
properties.
The polymeric compositions forming the anti-reflection coatings for the
optical devices of the present invention may also be described as being
made up of a polymer chain composed of
##STR2##
units wherein s, t and u have the meanings given above in connection with
formula (I), and wherein
X represents monomer-derived units of the composition
##STR3##
wherein R.sup.1, p and n, which may be the same or different in individual
X units within the polymer chain, have the meanings given in connection
with formula (I), above;
Y represents monomer-derived units of the composition
##STR4##
wherein R.sup.2, which may be the same or different in individual Y units
within the polymer chain, has the meaning given in connection with formula
(I), above; and
Z represents monomer-derived units of the composition
##STR5##
wherein R.sup.3 and R.sup.4, which may be the same or different in
individual Z units within the polymer chain, also have the meanings given
in connection with formula (I), above.
In the polymeric compositions of formula (II), above, the X, Y and Z units
may be arranged in any sequence. This freedom of arrangement accordingly
also prevails for formula (I), above, since formulas (I) and (II) are
merely alternate expressions for the same polymeric compositions.
These copolymers can be prepared by polymerizing the monomers in
tetrahydrofuran ("THF") or glacial acetic acid at elevated temperature
with a free-radical generating initiator, using procedures conventionally
employed in making acrylic and methacrylic polymers.
These copolymers are generally optically clear, without haze or
inhomogeneities. They have refractive indexes below about 1.4, generally
within the range of from about 1.365 to below about 1.4; good adhesion to
glass, silicon, copper foil, polyimide, nylon, polyethylene terephthalate,
polytetrafluroethylene, polychlorotrifluoroethylene and other similar
substrates; low surface energy, about half that of
polytetrafluoroethylene; excellent thermal stability in air; in
combination with good mechanical properties--they are neither brittle nor
elastomeric. They are soluble (up to about 40 percent by weight of the
combined weight of polymer and solvent) in about 1:1
THF/1,3-bis(trifluoromethyl)benzene (hereinafter also referred to as
hexafluoroxylene). From such solutions, coatings can be applied to any
suitable substrate, particularly optical substrates, such as glass,
polymers, polymer films, crystals, and the like. Their dielectric constant
is on the order of about 3.
It is an important feature of these copolymers that they can be
cross-linked by heat treatment without the use of cross-linking agents.
Such heat-induced cross-linking can occur either through internal
anhydride formation between two internal carboxyl groups situated on
pendant groups of monomer-derived moieties; or by internal esterification
between hydroxyl and carboxyl groups. Heat-induced cross-linking has the
advantage that no cross-linking agent is required, so that no impurities
are introduced; the cured polymer is a single component with no residual
solvent, monomer or cross-linking agents. The cross-linking process is not
associated with creation of large voids which can establish optical
scattering sites in the polymer. Such cross-linking improves hardness,
scratch resistance and adhesion of the polymer film, without change in
refractive index, and without deleterious effect on any other desirable
property. Heat treatment within the temperature range of from about
130.degree. C. to about 150.degree. C. for time periods of from about 0.25
to about 10 hours, desirably of from about 1 to 4 hours, results mainly in
esterification; heat treatment at higher temperatures, say within the
range of from about 170.degree. C. to about 180.degree. C., results in
significant anhydride formation. As a general proposition, higher
temperatures and longer heat treatment times tend to promote anhydride
formation. Cross-linking agents may also be employed, if desired, as to be
discussed in further detail below.
The unique properties of these copolymers which make them so eminently
suitable for use as anti-reflection coatings for optical devices are due
to the presence of the fluorinated moiety in combination with moieties
bearing carboxyl groups and moieties bearing hydroxyl groups. The
fluorinated moieties provide the desirable properties of fluoropolymers,
and the combination of the carboxyl groups and the hydroxyl groups
provides for processability and curability, properties which are typically
lacking in conventional fluoropolymers.
Anti-reflection coatings of the above-described copolymers are conveniently
applied to optical substrates, typically in 1/4 wavelength thickness, by
coating the substrate with a solution of the copolymer, removing excess
solution, if any, drying by evaporating the solvent, preferably, but not
necessarily, followed by heat-treatment, as above described, to cure the
coating by means of cross-linking. Typical substrates include optical
lenses; eyeglasses, both plastic and glass; windows, glass as well as
polymeric windows, such as windows of clear polymeric vinyl (incl.
copolymers thereof), styrene, acrylics (Plexiglass) or polycarbonate
(Lexan.RTM. supplied by General Electric); clear polymer films such as
vinyl (incl. copolymers), nylon, polyester, and the like; the exterior
viewing surface of liquid crystal displays, cathode ray tubes (e.g. video
display tubes for televisions and computers); and the like; the surface of
glossy displays and pictures, such as glossy prints and photographs; and
the like. Determination of suitable coating thickness (generally 1/4
wavelength of the light of which reflection is to be minimized) is within
the ordinary skill of the art, but is further elucidated, infra.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention may be better understood with reference to the
annexed drawings, wherein
FIG. 1 represents a graph showing total reflectance from an uncoated and
coated sapphire (a 1000 Angstrom layer of the terpolymer was spun on both
sides of the substrate);
FIG. 2 illustrates construction of a transferable anti-reflection coating
comprising an adhesive-backed optically clear polymer film having an
anti-reflection coating;
FIG. 3 is a curve illustrating the response spectrum of a normal daylight
adjusted human eye;
FIG. 4 is a curve showing the reflectance detected by the human eye from a
reflective surface having an anti-reflection coating deposited thereon.
FIG. 5 is a graph illustrating measured single surface reflectance (100-%
transmission) vs. wavelength for an uncoated, single coated and double
coated glass substrate.
FIG. 6 represents a graph showing the total reflectance from an uncoated
and coated black and white glossy photographic print having a uniform
black image (1000 Angstrom layer of the terpolymer was dip coated onto the
photograph).
DETAILED DESCRIPTION OF THE INVENTION
The following detailed description sets forth the preferred embodiments and
the best mode presently contemplated for its practice.
Regarding the copolymer composition, with reference to the "X" units of
formula (II), above, which are in more detail defined by formula (III),
above, these are derived from fluorine-containing acrylate or methacrylate
monomers of the formula
CH.sub.2 .dbd.CR.sup.1 CO--O--(CH.sub.2).sub.p --C.sub.n F.sub.2n+1
wherein R.sup.1, p and n have the meanings given above in connection with
formula (I). Those monomers wherein p is 2 are commercially available, as
mixtures of homologues having perfluoroalkyl groups of varying chain
length, that is to say as mixtures differing in "n", as they are usually
obtained in commercial manufacturing operations. Of course, one could
separate out individual compounds of defined perfluoroalkyl chain length,
if it were desired for any reason. For use in the anti-reflections coating
of the present invention, it is preferred to use monomers having a wider
distribution of "n", since such wider distribution makes for better
amorphicity, hence greater optical clarity, as will the use of acrylates
(wherein in the above formula R.sup.1 is H) with methacrylates (wherein in
the above formula R.sup.1 is CH.sub.3). Those monomers wherein p is 1 can
be readily prepared using known procedures. Preferably, p is 2 and n is an
even number. In preferred embodiments, n ranges from about 2 to about 30,
more preferably from about 4 to about 20. Specific examples of preferred
embodiments are the products sold by DuPont under its "Zonyl" trademark,
e.g. Zonyl TM (the methacrylate) and Zonyl TA-N (the acrylate), and sold
by Hoechst-Celanese under its "NUVA-HF" trademark. Such specific examples
include mixed perfluoroalkyl alkyl acrylates and methacrylates wherein n
is predominantly an even number, and in particular wherein the
perfluoroalkyl group is represented by a mixture of C.sub.4 through
C.sub.20 groups, particularly C.sub.6, C.sub.8, C.sub.10 and C.sub.12
groups.
The "Y" units of formula (II), above, which are in more detail defined by
formula (IV), above, are derived from acrylic acid, methacrylic acid,
itaconic acid, or mixtures thereof. All of these are commercially
available products.
The "Z" units of formula (II), above, which are in more detail defined by
formula (V), above, are derived from acrylic acid esters of the formula
CH.sub.2 .dbd.CR.sup.3 CO--O--R.sup.4 --OH
wherein R.sup.3 and R.sup.4 have the afore-stated meanings. In more
preferred embodiments, R.sup.3 is H or --CH.sub.3, with --CH.sub.3 being
most preferred. If R.sup.3 is represented by --CH.sub.2 C.sub.m
H.sub.2m+1, then m is preferably an integer of from about 0 to about 6,
more preferably of from about 1 to about 4. With respect to the R.sup.4
alkylene bridging group, embodiments having from 2 to about 4 carbon atoms
are preferred, as are the linear and branched chain embodiments. Use of
mixtures of such monomers of differing carbon-carbon chain length is
contemplated. To enhance amorphicity, use of mixtures of such monomers of
differing carbon-carbon chain length is desirable and preferred. Many of
the esters suitable for furnishing the "Z" units of formula (II), above,
are commercially available; those not so available are readily prepared by
those skilled in the art, using well-known procedures.
With regard to the weight proportions of the "X", "Y" and "Z" units (see
formula II, above), s ranges from about 0.5 to about 0.99, and t and u,
which may be the same or different, each range from about 0.005 to about
0.495. The preferred range for t+u is from about 0.02 to about 0.35, with
values in the range of from about 0.08 to about 0.3 being more preferred
yet. As to the weight ratio between t and u (t:u), weight ratios in the
range from about 1:0.5 to about 1:1.5 are preferred, with ratios in the
range of from about 1:0.8 to about 1:1.2 being more preferred yet.
Polymeric compositions of the present invention containing approximately
equal proportions by weight of the "Y" and "Z" components have been shown
to have desirable properties. If it is contemplated to subject the
polymeric composition to heat-induced cross-linking, as is preferred to
obtain more robust anti-reflection coatings, then the Y and Z components
are desirably employed in about equimolar proportions (rather than in
about 1:1 weight ratio). If equimolar proportions are employed, then the
cross-linking process, as above described, proceeds predominantly by the
internal esterification route, with minimal anhydride formation. The
esterification route is preferred because of the better stability of the
resultant product in high temperature and humid environments.
Polymerization of the monomers to make the polymeric compositions for the
anti-reflection coatings of this invention proceeds readily in solution in
THF or glacial acetic acid, at elevated temperature within the range of
from about 35.degree. C. to the boiling point of the polymerization
mixture, more desirably within the range of from about 45.degree. C. to
the atmospheric pressure boiling point of the solvent, about 65.degree. C.
for THF and about 110.degree. C. for glacial acetic acid, under autogenous
pressure, typically atmospheric pressure, using a free radical generating
initiator, such as 2,2'-azobis(2-methylpropanenitrile) (CAS #78-67-1)
available from DuPont under the designation VAZO 64, hereinafter referred
to as "AIBN". Other suitable initiators include
2,2'-azobis(2,4-dimethylpentanenitrile) (CAS #4419-11-8) and
2,2'-azobis(2-methylbutanenitrile) (CAS #13472-08-7). The
2,2'-azobis(2-methylpropanenitrile) is preferred.
The catalyst is employed in amount of from about 0.15 to about 0.4 percent
by weight, based on the combined weight of all the monomers to be
polymerized. Desirably, polymerization is conducted under agitation.
Typical polymerization times range from about 4 hours to about 8 hours.
The monomer concentration in the reaction medium typically ranges from
about 25 to about 50 percent by weight, based on the combined weight of
reaction medium (THF or glacial acetic acid) and the monomers.
Upon conclusion of the polymerization reaction, the polymer product is
readily recovered from the reaction mixture, as by evaporation of the
solvent and/or cooling the mixture to precipitate the polymer product,
followed by separation of liquid and solid phases, as by filtration, and
washing of the polymer product to remove residual unreacted monomers using
any suitable solvent, if desired. These operations are conventional. Once
recovered and purified, as here described, the polymer product seems to be
insoluble in the polymerization mixture. However, it is nicely soluble in
about equal volumes of THF and hexafluoroxylene, in concentrations of up
to about 40 percent by weight, based on the combined weight of polymer
product and solvent. Neither THF nor hexafluoroxylene by itself dissolves
the polymer product. Solution of the polymer product in the mixed solvent
is aided by mild heating and agitation.
The appended claims are intended to cover anti-reflection coated optical
devices wherein the polymeric compositions contain incidental amounts, say
up to about 10% by weight of other comonomers, and particularly of acrylic
esters, which do not interfere with the polymerization, and which do not
deleteriously affect desirable properties of the polymer product. Examples
of such incidental, additional monomeric materials include alkoxy alkyl
acrylates and alkoxy alkyl methacrylates (such as methoxy, ethoxy,
propoxy, butoxy and higher acrylates and methacrylates); epoxy alkyl
methacrylates; alkyl acrylates and methacrylates, including haloalkyl
derivatives thereof, such as chloroalkyl acrylates and methacrylates; and
the like.
When the ratio of the Y-component (acid component) to the Z-component
(hydroxyl- bearing acrylic ester) in the polymeric composition of this
invention is larger than 1.0, then the preferred curing product is the
anhydride. When the ratio is smaller than 1.0, an ester is the preferred
product, with some hydroxyl groups remaining unreacted. When the ratio is
1.0, then the preferred product is the ester, with practically all the
hydroxyl groups being consumed.
Example 1, below, illustrates typical polymerization procedure.
Example 1
Typical polymerization procedure for perfluoroalkylethylacrylate terpolymer
A 500 ml 3-neck round bottom flask containing a large magnetic stirring
"egg" is immersed in a thermostated oil bath on a stirring hot-plate. A
stream of dry nitrogen is introduced through one of the necks to keep the
polymerization mixture under nitrogen atmosphere throughout the
polymerization. Another neck is stoppered. Through this neck the
polymerization initiator is added by momentarily opening the stopper. The
third neck is equipped with a pressure-equalizing dropping funnel. On top
of this dropping funnel, a long vigeraux condenser is placed equipped on
the top with a very narrow exit. Extra solvent is placed in the dropping
funnel, which is dropped into the polymerization vessel in order to
compensate for loss of solvent which may arise from the combination of
nitrogen flow and elevated reaction temperature. With the above
arrangement, each drop of solvent (which can also be a solution of a very
reactive monomer in the same solvent) is swept by the nitrogen flowing in
the opposite direction, i.e., by the dropping exit of the dropping funnel,
on top of the solvent in this funnel, up through the vigeraux column and
out through the narrow outlet.
The monomers are purified from any polymerization inhibitors that may be
present, and the desired proportion is weighed into the round bottom
flask. 72.10 g perfluoroalkylethylacrylate monomer mixture
(Hoechst-Celanese NUVA FH), 7.34 g acrylic acid and 7.35 g
2-hydroxyethylacrylate are weighed into the flask. Their percentage
weights are 83.09% NUVA FH, 8.46% acrylic acid and 8.45%
2-hydroxyethylacrylate. To this mixture, 105 ml tetrahydrofuran (THF) is
added. The solution volume should be about half the volume of the round
bottom flask. The mixture is stirred under nitrogen flow with very slow
heating to about 40.degree. C. for about an hour. Then there are added to
the clear solution 0.27 g (0.3% by weight based on total monomer weight)
of 2,2'-azobis(2-methylpropanenitrile) (AIBN) (duPont's VAZO 64), a
convenient polymerization initiator. The polymerization mixture is slowly
brought up to around 62.degree. C. and the polymerization is allowed to
proceed for several hours. Then the mixture is poured into a
crystallization dish and most of the THF is stripped off. The residual
materials are washed and comminuted in methanol, the unreacted monomeric
species are washed away and the powdery white polymer is dried under high
vacuum at temperatures not exceeding 50.degree. C. The reduced viscosity
of a polymer thus prepared was determined in a glass viscometer at
25.degree. C. on a 2% solution of the polymer in 1:1 vol/vol mixture of
THF and 1,3-bis(trifluoromethyl)benzene. A value of .eta..sub.red =0.15
dL/g was obtained. NMR analysis indicated that the monomer composition in
the terpolymer was extremely close to the feed composition. After spinning
on silicon wafers and on glass substrates, refractive indices of n=1.3885,
n=1.3890 and n=1.3865 were measured on three samples taken at random from
a large population.
For the purpose of spin coating, mixtures of THF and
1,3-bis(trifluoromethyl)benzene in the range of 1:3 up to 3:1 were found
to be the only acceptable ones, with the most desirable being a 1:1
vol/vol mixture. No other solvent or solvent mixture from which acceptable
coatings could be spun using commonly available spinners (e.g., Headway
Research, Inc., Photoresist Spinners) under ambient temperature and air
humidity conditions has been found.
Preparation of perfluoroalkylethylmethacrylate terpolymer follows the
procedure set forth above in Example 1. In general, the intrinsic
viscosities of the methacrylates are higher than those of the acrylates,
i.e. in the order of about 0.25 dl/g vs. 0.13-0.15 dl/g. THF and glacial
acetic acid were found to be suitable solvents to conduct the
polymerization. However, two additional solvents were found for the
methacrylic monomer mixture polymerization. These are 4-methyl-2-pentanone
(MIBK) and 2-butanone (MEK). These solvents are not as good as THF for the
purpose of the polymerization and MEK is poorer than MIBK. In MIBK and MEK
the polymerizations are conducted at temperatures up to about 80.degree.
C., substantially above the 65.degree. C. boiling point of THF. Workup is
the same as for the acrylic polymers.
The above mentioned mixture of THF and 1,3-bis(trifluoromethyl)benzene in a
preferable ratio of 1:1 vol/vol. ratio is a suitable solvent for spinning
or spray-coating the polymers of this invention. A large number of
solvents and solvent mixtures were tested for the purpose of spin coating,
but only one solvent mixture was found to perform satisfactorily with
respect to the high polymer solubility required for spin coating, and the
evaporation rate of the solvent after the solution was spun on the
substrate, namely the THF/1,3-bis(trifluoromethyl)benzene mixture in
ratios of from 1:3 to 3:1, preferably in ratio of about 1:1, by volume.
A typical spin-coating and thermal cross-linking (curing) procedure for
perfluoroalkylethyl terpolymer is described in Example 2, below.
Example 2
This example illustrates a coating application employing a fluoropolymer
composition of this invention.
The polymer is dissolved in the solvent mixture at a concentration
producing a solution of sufficient viscosity for conventional spin
coating. For the modest molecular weight polymers in this invention, a
polymer concentration of about 20 wt/vol % is usually employed. A few
drops of the solution are applied to the center of the substrate (e.g.
optical glass) and the system is spun in the spinner for about 30 seconds
or less. The speed of spinning varies from ca. 2000 rpm up to over 5000
rpm.
When the spinning process is complete, the coated substrate is removed from
the spin-head, placed in an oven and cured at temperatures of 130.degree.
C. up to 200.degree. C., preferably between about 130.degree. C. and about
180.degree. C., for 1 to 4 hours. A self-crosslinked coating is obtained.
The coating is tenaciously attached to the substrate and is not removed by
scratching, masking-tape crosshatch procedure, immersion in solvents or
acids, etc. For systems containing over ca. 75 wt. %
perfluoroalkylethyl-(meth)acrylate comonomer, the refractive index is, as
a rule, less than 1.400.
As previously indicated, the polymeric compositions of this invention can
also be cross-linked employing conventional cross-linking agents, such as,
for example, diisocyanates, carbodiimides, diacid chlorides, and the like.
Examples of specific effective crosslinking agents include
hexamethylenediisocyanate, methylene di-p-phenyldiisocyanate,
1,3-dicyclohexyl carbodiimide, dodecanedioyl dichloride and adipoyl
chloride. The crosslinking agents are employed in amounts conventionally
employed to obtain desired cross-linking of the polymer which, by use of
such agents, can take place at ambient temperatures.
To be effective in suppressing the undesired reflection, the
anti-reflection coating should have a refractive index less than that of
the substrate, or the underlying layer on which the coating is applied,
and have the appropriate optical thickness. The optical thickness is
defined as the physical coating thickness times the material's refractive
index. According to the conventional theory of reflection for dielectric
interfaces, the reflectivity for normally incident light is given by
##EQU1##
Therefore, in order to achieve zero reflectivity (numerator set to 0), the
ideal coating refractive index is equal to the square root of the
refractive index of the substrate times the square root of the refractive
index of surrounding medium. In most applications, this surrounding medium
is air, which has a refractive index of 1. Therefore, if the refractive
index of the coating material is exactly equal to the square root of the
substrate refractive index, all surface reflection will be eliminated at
the wavelength corresponding to 4 times the optical thickness. At other
wavelengths, while the destructive interference from the reflected light
from the top and bottom coating interfaces will not be complete,
substantial reduction in reflection will still be obtained. For most
applications, the optimal anti-reflection coating can be obtained by
making the optical thickness one quarter of the mid-point of the visible
wavelength range (one quarter of 5500 Angstroms or about 1400 Angstroms).
It should be noted, however, that in certain circumstances, it may be
desirable to reduce the reflection in a certain portion of the spectrum
other than the midpoint. This can easily be done by slightly altering the
process parameters.
The substrates described in this patent application have a refractive index
of at least 1.49. Ideally, the refractive index of the coating material
should fulfill the square root requirement mentioned above. For example,
to optimally coat standard window glass, which has a refractive index of
about 1.5, the coating material should have a refractive index of about
1.23. To coat many polyesters, which have a refractive index of about
1.66, the coating material should have a refractive index of about 1.29.
While the terpolymers do not approach this ideal, their refractive index
is sufficiently low to make them useful for anti-reflection coating
applications. As an example of this, uncoated substrates with a refractive
index of 1.5 have a reflectance of about 4% per surface at normal
incidence. Preferably, reflections below 2% are desired, which corresponds
to a refractive index of less than 1.41; more preferably, reflections
below 1.5% are desired, which corresponds to a refractive index of less
than 1.38.
Although single layer coatings are sufficient for many applications, they
do have limitations. For example, the minimum reflectance, per surface,
obtained by using a single terpolymer layer on crown glass is about 1.5%.
In many circumstances, this may be unacceptably high. It is possible to
even further reduce, and often completely eliminate, the minimum surface
reflection by using multi-layer anti-reflection coatings, specifically two
layer coatings. One of the trade-offs, however, is that the reflection
rises rather sharply away from the wavelength of minimum reflectance. The
creation of two layer coatings involves the application of a high
refractive index layer onto the substrate surface, then the subsequent
application of a low refractive index layer on top. The refractive index
of the high index layer must be greater than that of the substrate, while
the refractive index of the low index layer must be lower than the
substrate. It should be noted that solvent selection is extremely
important, so that they are compatible with the substrate and do not
redissolve the other coating layers.
Those skilled in the art will realize that unlike with single layer
coatings, the thicknesses of each layer in a two layer coating can be
modified over a relatively wide range to produce the desired
anti-reflection coating. The optical thickness of high index layer is
preferably about one quarter or one half wavelength, while the optical
thickness of the low index layer is preferably about one quarter
wavelength. In the most conventional two layer coating, each layer has an
optical thickness of one quarter wavelength. Ideally, in this case, the
coating materials are chosen such that the square of the refractive index
of the high index material divided by the square of the refractive index
of the low index material is equal to the refractive index of the
substrate. If this is not possible, it is preferable that the index
difference between each layer and the substrate be at least 0.1.
Quantitative determination of the reflection properties of multilayer
dielectric coating are well understood and known to those skilled in the
art.
The refractive index of the anti-reflection coatings can be determined as
follows: A 15-20 wt % solids solution of the polymer in an appropriate
solvent is spin cast onto a clean silicon wafer at rotation speeds between
1500 and 3000 rpm. In general, this will yield a film thickness between 1
and 3 microns. The fluoropolymer coating is then cured in a convection
oven at 150.degree. C..+-.20.degree. C. for 4 hours. However, it was found
that the refractive indexes were unaffected with cure times as short as 10
minutes and as long as 24 hours. The room temperature refractive indexes
of the polymers are then measured at 632.8 nm using a Metricon.RTM.
PC-2000 Prism Coupler.
The substrates used for anti-reflection coated devices include, but are not
limited to, two major categories: inorganic oxides and plastics. Typical
inorganic oxides include, but are not limited to, fused quartz, glass (all
grades of optical glass as well as any and all common varieties), and
sapphire.
Typical substrates include optical lenses; eyeglasses, both plastic and
glass; windows, glass as well as polymeric windows, such as windows of
clear polymeric vinyl (incl. copolymers thereof), styrene, acrylics (such
as Plexiglass) or polycarbonate (Lexan.RTM. supplied by General Electric);
clear polymer films such as vinyl (incl. copolymers), nylon, polyester,
derivatized cellulose, and the like; the exterior viewing surface of
optical (electro-optical) flat panel displays, such as liquid crystal
displays of all types, ac plasma displays, dc gas discharge displays,
electroluminescent displays, light emitting diodes, vacuum fluorescent
displays, and the like; cathode ray tubes (e.g. video display tubes for
televisions and computers) and the like; the surface of glossy displays
and pictures, such as glossy prints and photographs, and the like; and
optical indicator components, such as dials, knobs, buttons, windows and
the like in environments where reflections are a problem such as aircraft
interiors, aircraft cockpits, automotive interiors and the like.
The following describes typical procedures for applying the anti-reflection
coatings to make the devices of the present invention. This description is
illustrative only, and subject to modification from case to case to
optimize coating quality and/or to accommodate different materials, as is
within the skill of the art.
Prior to the coating operation, the substrates are scrubbed clean with
methanol in an ultrasonic cleaner for at least 30 seconds. Upon removal,
they are sprayed with fresh methanol to insure that no contamination
remains on the surface. After being blown dry with filtered nitrogen air,
the substrates are baked in an convection oven for about 5 minutes at
about 100.degree. C. to remove any residual moisture. No additional
surface treatment steps are necessary before applying the coatings.
The substrates may be coated either by spin coating or dip coating from
solutions of the polymers described above. Flexible substrates, such as
nylon or polyester (Mylar) films are preferably dip coated. Rigid
substrates may be coated using both methods. The polymer solution
concentrations needed for these applications varied depending upon the
specific polymer, its molecular weight, and the solvent used. In general,
workable polymer concentrations are in the range of 0.5 wt % to 2 wt %
solids for spin coating and 3 wt % to 8 wt % solids for dip coating. It
should be noted that for dip coating, polymer concentration variations of
0.1% were found to alter the thickness of the film on the order of 100
Angstroms.
The dipping can be performed using a Newport Corporation Actuator 850
motorized micrometer attached to a translation base, typically employing a
stage range of 1 inch. The actuator may be controlled by a Newport
Programmable Controller 855C. Substrates are dipped into and pulled out of
polymer solution at rates between 100 and 400 microns/second, where faster
pulling rates correspond to thicker films. As an approximate rule, the
thickness of the pulled film increases linearly with the pulling rate. In
our operation, the polymer solution was contained in a vial that was, at
most, half full. This allowed for the upper half of the vial to have a
semi-solvent atmosphere, giving the film time to dry slowly. Dip coating
needed to be done in an area with no drafts, since the air currents tended
to create streaks on the substrates by causing substrate motion and
inhomogeneous drying. Unless special precautions are taken, dip coating
yields polymer films on both sides of the substrate.
The spin coating may be performed using a Headway Research photoresist
spinner. In our operation, the spinner was enclosed in a Plexiglass box
with a laminar flow hood mounted on top. Filtered nitrogen air was used to
purge the spinning chamber and keep it reasonably dust-free. Samples were
spun cast at rotation speeds between 1500 and 3000 rpm.
All initial samples on non-polymeric substrates were cured in a convection
oven at 150.degree. C. for 4 hours. It was found, as with the refractive
index measurements, that curing times ranging from 10 minutes to 24 hours
did not affect the overall optical properties of the films. Subsequent
samples were therefore cured at temperatures between 100.degree. C. and
150.degree. C. for up to 1 hour. For two-layer films, the initial layer
was cured for at least half an hour before the second layer was coated. It
will be recognized that other curing means can be employed, including
infrared lamps, hot bars, microwave radiation, infrared lasers, as well as
other sources of thermal stimulation.
The thicknesses of the anti-reflection films were measured using two
different methods. For glass substrates, a Sloan Dektak IIA profilometer
was used not only for the thickness measurements, but also for an
evaluation of the surface roughness. A measure of the thickness could also
be inferred from the transmission in the spectral data of the
anti-reflection coated sample. Since there was a high degree of
correlation between the theoretical model and the experimental data, it
was possible to obtain a highly accurate thickness measurement by matching
the experimental wavelength of minimum reflection with the theory. This
latter method was used exclusively with the plastic substrates.
All transmission and reflection measurements of antireflection coated
samples were done using a Perkin-Elmer Model 330 spectrophotometer. The
normally incident transmission measurements were done relative to air.
Many of the plastic substrates contained dyes, microcrystallites, or
surface machine grooves that were not removed after manufacturing. It was
therefore often difficult to infer absolute reflectivity from the
transmission spectra. Reflectance spectra were taken relative to a freshly
aluminized quartz slide and calibrated by measuring the reflectance from a
clean quartz slide. Due to the geometry of the apparatus, the probe beam
had a 6. angle of incidence on the sample.
Glass (of any type, incl. optical glasses as well ordinary window glass),
quartz, and oxide crystals, such as sapphire, are rigid substrates that
are impervious to all organic solvents. They are, therefore, the most
easily processable. Anti-reflection coated samples are conveniently
prepared via either spin coating or dip coating, followed by curing at
elevated temperature. Since sapphire has a very high refractive index, the
amount of reflected light can be significantly reduced by simply using a
single layer anti-reflection coating with materials having a refractive
index of approximately 1.38. A 1000 Angstrom thick film of above-described
terpolymer was spin coated onto both sides of a sapphire substrate and
fully cured at 160.degree. C. for four hours to allow for full
crosslinking. Transmission spectra were taken for both the coated and
uncoated samples. The results are shown FIG. 1.
Unlike sapphire, both quartz and microscope glass have relatively low
refractive indexes. While the reflection can be significantly reduced by
applying a single terpolymer layer of the appropriate thickness, it may be
advantageous to use a two layer coating. As stated earlier, the bottom
layer consists of a high refractive index material and the top layer
consists of a low refractive index material. The words "high refractive
index" and "low refractive index" are referenced relative to the
refractive index of the substrate.
In general, each layer in a two layer anti-reflection coating can have a
rather wide range of thicknesses. In the most common embodiment, however,
each layer has an optical thickness of one quarter wavelength. The high
index layer can be made from a wide range of materials, such as poly
(9-vinyl carbazole) which has a refractive index of approximately 1.67.
This polymer readily dissolves in many common solvents, such as
cyclohexanone.
As an example of the advantages of two layer coatings, consider a substrate
with a refractive index of 1.5. The bare substrate would have a
reflectance of 4% per surface at normal incidence. If a single layer
coating, with a refractive index of 1.38, were applied at a quarter
wavelength optical thickness, the reflectance would decrease to 1.4% per
surface at 5500 Angstroms. If the substrate had a two layer coating, low
index layer with a refractive index of 1.38 and high index layer with a
refractive index of 1.67, both with an optical thickness of one quarter
wavelength, the reflectance would decrease to 0.03% per surface at 5500
Angstroms.
The anti-reflection coating process described above can also be applied to
polymeric materials. In particular, materials that do not dissolve or
swell in the solvent systems used for the coating materials, such as cured
epoxies, cured polyurethanes, etc. can be readily coated. Other polymeric
materials such as Lexan.RTM. (polycarbonate), plexiglass (thermoplastic
acrylics), and the like may be dissolved or swelled by the mixed solvent
system described above. In these cases, an alternate coating mechanism
will need to be employed, such as the use of a different solvent system,
melt processing, or water based emulsion. In general, when the solubility
parameter .delta. of the solvent or solvent mixture of the coating is
different from (either larger or smaller) the measured or calculated
solubility parameter of the substrate by more than about 1 (MPa).sup.1/2,
then the solvent mixture will not dissolve the substrate.
For example, the .delta..sub.solvent (MPa).sup.1/2 for
tetrahydrofuran (THF)=18.6,
1:1 THF/hexafluoroxylene (HFX)=17.2.
Accordingly, the following solubilities will be observed:
______________________________________
Solvent Substrate .delta..sub.substrate
Result
______________________________________
THF/HFX PET 21.9 no effect
THF/HFX nylon 66 27.8 no effect
THF/HFX PTFE 12.7 no effect
THF PVC 19.3 soluble
THF/HFX PVC 19.3 insoluble
THF polystyrene 17.6 marginally soluble
THF poly(methyl 18.6 highly soluble
methacrylate)
THF PTFE 12.7 insoluble
THF polyvinyl alcohol
25.8 insoluble
______________________________________
Incidentally, the solubility parameter .delta. for a terpolymer as above
described, wherein s:t:u=80:10:10 is 17.3, so that that terpolymer is
soluble in 1:1 THF/HFX.
Both single layer and two layer effective anti-reflection coatings were
made on flexible polymer films via dip coating. The substrate materials
included nylon 6, and polyester (Mylar).
Exemplary optical devices of the present invention are eyeglass lenses and
liquid crystal displays provided with an anti-reflection coating of the
above-described terpolymer, applied as herein described.
Example 3
Most common eyeglass lenses are made out of glass (n=1.51) or a hard
crosslinked polymer resin (n=1 51), although there is a move toward higher
refractive index materials to reduce the required curvature. In both
cases, the materials do not dissolve in any of the solvents used for the
above-described polymer compositions. Both single layer and two layer
anti-reflection coatings were applied to both types of lenses via spin
coating. Transmission spectra were taken for both uncoated and coated low
positive diopter lenses. We observed about 97% transmission for a single
layer coated lens and greater than 100% transmission for a two layer
coated lens, both at their respective design wavelengths (the wavelength
of minimum reflection). In comparison, an uncoated lens exhibited about
92.5% transmission. The percentages are slightly elevated relative to flat
substrates due to lensing of the probe beam. There were negligible
differences between the glass and resin lenses.
Example 4
Both single layer and two layer anti-reflection coatings were applied to
liquid crystal displays via dip coating. In particular, we used a 4 digit,
24 pin device. To ensure that the pins were not covered with polymer, they
were encapsulated in a thick layer of rubber cement. The cement did not
appear to dissolve in tetrahydrofuran, hexafluoroxylene, or cyclohexanone.
The first LCD was dip coated in a solution of a terpolymer composition as
above described and cured at 100.degree. C. for 30 minutes. The second LCD
was initially dipped in polyvinylchloride solution (.about.5% by weight
PVC in cyclohexanone), baked at 100.degree. C. for 60 minutes, then in the
terpolymer solution and again cured at 100.degree. C. for 30 minutes. The
rubber cement was then peeled off. Visually, there was a marked decrease
in the reflected light. There was also the characteristic bluish color
reflected from the LCD with a two layer coating. This bluish hue is the
consequence of the rather high reflectivity in the blue and the
comparatively low reflectivity throughout the remainder of the visible
spectrum.
Example 5
When a glossy photograph is viewed under normal lighting conditions, a
small, though significant, portion of the light is reflected back, making
the color and the image appear "washed out". To minimize this problem, it
is usual practice to use a mat surface finish. This, however, reduces the
resolution of the image. We have demonstrated that an anti-reflection
coating of a thin layer of the above-described terpolymer when applied to
a glossy photograph significantly reduces specular reflection. A glossy
black and white photograph containing a uniformly black image was
partially coated with the terpolymer described above via dip coating. The
total reflectance, both specular and-diffuse, were measured using a Perkin
Elmer 330 spectrophotometer equipped with an integrating sphere.
Measurements were perform on both the uncoated and coated sections of the
photograph at a 7.degree. angle of incidence. The data, shown in FIG. 6,
demonstrates the substantial decrease in reflectance.
A further effective application of the anti-reflection coated optical
devices of the present invention involves their use as transparent covers
for read-out instruments and instrument panels, such as automotive
instrument panels. Such panels are commonly tilted, or curved, to reduce
back reflected light. Tilting or curvature tend to reduce overall
visibility, and to increase the size of the component. Application of an
anti-reflection coating of the above-described terpolymer composition
effectively reduces reflection and increases instrument visibility.
Besides spin coating and dip coating, as above described, the
anti-reflection coating may also be applied by spray coating and roller
coating. In the former, a fine mist of polymer solution is sprayed onto
the substrate in a semi-solvent atmosphere and allowed to dry slowly.
There are numerous parameters that need to be controlled in order to use
this technique: solution viscosity, mist particle size, substrate movement
speed, spray area overlap, and sample orientation. In the latter, roller
coating, a squeege (similar to that used in screen printing) is used to
apply a uniform thin polymer layer.
There are some difficulties in applying a thin polymer layer of uniform,
predetermined thickness to large curved objects. A transferable
anti-reflection coating could provide a solution. In accordance with the
present invention, this is accomplished by the provision of structure
(here referred to as "applique") comprising an optically clear plastic
film, such as polyester (Mylar) which is anti-reflection coated on one
side, with an refractive index matching adhesive on the other side, as is
illustrated in FIG. 2. As shown in FIG. 2, an optically clear plastic film
1 has an anti-relection coating 2 applied to one side, and an adhesive
coating 3 on the opposite side. The adhesive coating 3 is backed with
peel-away release film 4, which may be conventional release-coated paper.
For application, the release film is peeled off, and the anti-reflection
coated highly transmissive optical film is applied to the viewing surface
of flat surface displays (as enumerated, supra.), CRT's, VDT's,
eyeglasses, etc. This alleviates the problem of trying to coat cumbersome
objects. If damaged, the applique can easily be removed and replaced.
Preparation of such an applique is illustrated by Example 6, below:
Example 6
Cleer-Adheer.RTM. sheet (C-Line Products, Des Plaines, Ill.) was used as
the base for an applique, The sheet consists of a mylar film backed with a
pressure-sensitive acrylate adhesive and peel away release paper. The
entire laminating sheet was dip coated with a solution in
tetrahydrofuran-hexafluoroxylene of above-described terpolyer and baked at
100.degree. C. for 30 minutes. The solvent mixture very slowly dissolved
some of the adhesive. That portion of the film was cut off. Visually,
there was a significant difference in light transmission and optical
clarity between the uncoated and coated sections.
Example 7
A 0.05 mm thick clear security Llumar Film (All Purpose Glass Coating Co.,
Clifton, N.J.), consisting of a polyester film backed with a pressure
sensitive adhesive and a peel-away plastic backing film, was used as the
base for an appliquee. A 20.times.25 cm sheet of this film was laid flat
on one side of a wet glass sheet. The water permitted easy movement of the
film until it was placed in the proper position. Using a soft cloth, the
water was pressed out, leaving an adhesive/glass interface. The same
process was repeated on the other side of the glass. The glass sheet so
treated on both sides was allowed to dry at room temperature for 24 hours,
after which time it was baked at 60.degree. C. for 4 hours. The resultant
coated glass sheet was clear and free from air bubbles. The surfaces were
cleaned with soap and water, followed by drying at 100.degree. C. for 10
minutes. Both sides of the laminated glass sheet were then dip coated with
a THF/HFX solution of above-described terpolymer and baked at 100.degree.
C. for 30 minutes. There was a significant enhancement of optical
transmission, as measured using a Perkin Elmer 330 spectrophotometer. The
average total reflectance near 5500 Angstroms for the coated, laminated
glass was approximately 4.5%, as compared to about 8.5% for the bare
glass. The clarity enhancement was not quite as great as that obtained on
a bare glass sheet coated with the terpolymer, possibly due to slight
index mismatches between layers.
Example 8
A film as described in Example 7, above, is dip coated with the THF/HFX
terpolymer solution employed in Example 7. It is permitted to dry and is
then baked at 125.degree. C. for 1 hour to cross-link the terpolymer.
Thereafter, the film is permitted to cool to room temperature, the backing
film is peeled away, and the film without the backing sheet is placed on a
wet glass sheet and properly positioned. The water is then squeezed out
using a soft cloth, and the structure is permitted to dry at room
temperature for 24 hours, followed by baking at 100.degree. C. for 4
hours. The laminated glass sheet thus obtained exhibits significant
optical clarity and increased light transmission.
Every application has its own glare reduction requirements. Since it is not
possible to attain zero reflectance across the entire spectrum, each
application must be analyzed to choose the optimal coating. The goal of
minimizing the total reflectance can be achieved by numerically
calculating the integral
##EQU2##
where TR is the total reflectance, R(.lambda.) is the spectral reflectance
vs. wavelength, I(.lambda.) is the spectral intensity
distribution vs. wavelength, S(.lambda.) is the spectral
sensitivity of the detector vs. wavelength, and .lambda. is the wavelength.
Ideally, the reflectance minimization is accomplished by reducing the
reflectance in the wavelength region where I(.lambda.)S(.lambda.) is
large.
The effect of the choice of detector is shown using data from the results
section. For most commercial applications, the detector is the human eye.
In FIG. 3, a sensitivity curve is shown corresponding to a normal
daylight-adjusted eye [M. Alpern, "The Eyes and Vision," in Handbook of
Optics, W. G. Driscoll et al., Eds., New York: McGraw-Hill, 1978]. Curves
corresponding to eyes that are night-adjusted or have ocular problems are
also available and may be appropriate for certain applications [ibid.]. We
assume that under ambient room lighting conditions, the light intensity
distribution is reasonably uniform across the entire visible spectrum.
Corrected reflectance curves, using the data from FIG. 5, are shown in
FIG. 4.
We have qualitatively evaluated the adhesion and mechanical properties of
the polymers. The degree of adhesion was measured using two different
methods. The first method involved using a pencil eraser to push the
polymer film off the substrate. The other method used was the standard
Scotch.RTM. tape test. For this, crosshatches were made in the film using
a razor blade. Scotch.RTM. tape was pressed onto the polymer and quickly
pulled off. The scratch resistance was determined by repeatedly wiping the
polymer film with a Kim-wipe.RTM..
The uncured fluoroterpolymer has relatively poor adhesion and durability.
Upon curing, however, it was not possible to remove the film using the two
tests mentioned above. In fact, the polymer could not be removed without
physically abrading or scratching the film. No noticeable scratches were
observed after wiping the film repeatedly.
Since various changes may be made in the invention without departing from
its spirit and essential characteristics, it is intended that all matter
contained in the description shall be interpreted as illustrative only and
not in a limiting sense, the scope of the invention being defined by the
appended claims.
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